Scoop the potatoes and garlic into a medium (3-quart) saucepan, pour in the broth and set over high heat. When the liquid boils, reduce the heat and simmer until the potatoes are tender, about 15 minutes.
While the potatoes are cooking, roast the poblanos over an open flame or 4 inches below a broiler, turning regularly until blistered and blackened all over, about 5 minutes for an open flame, about 10 minutes for a broiler. Transfer to a bowl and cover with a kitchen towel. Let cool until handleable. Rub the blackened skin off the chiles and pull out the stem and seed pod. Rinse the chile flesh to remove bits of skin and seeds. Place into a blender jar.
When the potatoes are tender remove from the heat and stir in the watercress. Once it’s wilted, carefully transfer some of the soup to the blender jar, taking care not to overfill since the mixture is very hot. You’ll need to blend in several batches, adding the chives and cilantro to each batch as you blend it to a smooth consistency. Pour the soup into a large bowl, stir in the milk and season with salt, usually 1 1/2 teaspoons. Refrigerate until cold. Ladle into chilled bowls, garnish with chopped chives, cilantro leaves and freshly grated lime zest.
Jill's Wine Recommendation: I love the 2009 Sitios De Bodega “Con Class” Verdejo from Rueda, Spain. It cut through the richness of the cream and brought out the citrusy lime zest.